What’s behind the recent rash of violent weather

Violent weather has tormented regions from the Rocky Mountains to the Mid-Atlantic in recent weeks. In Kansas Tuesday night, strong tornadoes tore houses apart, littered an airport runway with debris and hoisted a car onto a roof -- but widespread flooding may be the biggest and most prolonged threat. William Brangham talks to atmospheric scientist Victor Gensini about the brutal spring weather.

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Judy Woodruff:

As we've been reporting, severe weather pounded large sections of the country again today. Flooding is already overwhelming residents in Oklahoma and Arkansas, and is expected to worsen in the next few days.

Violent storms are claiming lives as well. So far, tornadoes have been blamed for at least 38 deaths this year.

As William Brangham tells us, tornadoes injured more people overnight when they battered parts of Kansas and Northwest Missouri.

William Brangham:

Several tornadoes struck Kansas overnight, with winds so powerful this car was thrown onto the roof of a house, and knocked this semi onto its side.

Dozens of homes were torn apart in Lawrence, Kansas, with at least a dozen injuries there as well. Debris blown from nearly 50 miles away left the runway of the Kansas City Airport so dangerous, it had to be shut down and evacuated for several hours last night.

Passengers were left stranded and ducking for cover.

Woman (through translator):

It came over the loudspeakers that everybody was to move, and then everybody just moved very quickly and orderly to the basement.

William Brangham:

Tornadoes also struck as far as Eastern Pennsylvania.

Woman:

All of a sudden, it is like rapid-fire machine gun hitting the side of the house.

William Brangham:

For the last two weeks, an unrelenting barrage of floods, tornadoes and extreme weather has pummeled the middle of the country.

And over the last month, the National Storm Prediction Center says there have been over 500 reported tornadoes. Today, 11 states and the District of Columbia still were given an enhanced risk advisory of severe weather.

For many communities, though, flooding remains the much bigger problem. Cities and town along the swollen Arkansas River in Oklahoma and Arkansas are preparing for that river to crest. In fact, within the next week or so, every large community along the Arkansas is expected to suffer major or record flooding.

Arkansas Governor Asa Hutchinson warned that levees could be breached.

Gov. Asa Hutchinson:

This is a flood of historic magnitude. It surpasses all Arkansas River flooding in our recorded history. That should be enough to get everybody's attention.

William Brangham:

And along the swollen Mississippi, eight states have now seen the longest stretch of continuous flooding since the Great Flood of 1927. And the forecast offers no relief. More rain is expected.

We spoke with meteorologists at the National Storm Prediction Center today, and they told us that the number of consecutive days we have had with tornadoes is a pretty rare occurrence, and certainly far more than in recent years.

To help us understand more, we turn to Victor Gensini. He's an atmospheric scientist at Northern Illinois University. And he joins us via Skype.

Victor, thank you very much for being here.

I wonder if you could just help us understand, why are we seeing so many tornadoes recently?

Victor Gensini:

We have been in an incredibly anomalous weather pattern, especially even for May standards.

We see a lot of tornadoes in May every year, but this particularly pattern is noted by a big jet stream in the western portion of the United States, and east of that jet stream, sort of like a roller-coaster type of pattern.

Moisture is being brought north out of the Gulf of Mexico, creating an atmospheric sort of cesspool of tornadoes across the center portion of the United States. And this has been going on now for — this is day 13.

William Brangham:

And are those the traditional underlying factors that cause tornadoes?

Victor Gensini:

Yes.

You would really like — if you're a forecaster and you're looking for tornadoes to occur, you're looking for very strong wind shear, which is provided by the jet stream, which is a narrow ribbon of wind up at where commercial aircraft fly, around 30,000 to 35,000 feet.

You really want to see that over top of really humid, unstable air at the surface. And, as that air begins to rise and the jet stream begins to tilt those updraft of the storms, you can get what we call supercell storms, which are rotating thunderstorms, to produce tornadoes.

And that's really what's been prevalent over the past few weeks.

William Brangham:

And we have also, as we have been just reporting on, seen some incredible flooding along some of the major rivers in the Midwest of the country, driven largely by huge rainfall.

Are those particularly unusual events for this time of year as well?

Victor Gensini:

Not only unusual, but also very strongly correlated to all the tornado activity that we have been seeing.

I know, when you start talking about economic loss, it's not just going to be the tornadoes and hailstorms that we have seen, but we will see way more from the widespread flooding. Farmers in the Midwest and my home state here of Illinois are so far behind on planting this year due to just the unbelievable, copious amounts of rainfall.

William Brangham:

There's also, with all of these conversations we have been having in recent years about extreme weather, how much climate change is driving this, or maybe how much climate change may be exacerbating the underlying conditions.

What does your research tell us about this particular set of storms?

Victor Gensini:

We can't say anything from a small subset, like just this May. We really like to look at weather and climate as two separate things.

So, if you imagine, for a baseball analogy, weather being an at bat and climate is your batting average, we have to look at the succession of these events over a 30-to-40-year period to really understand how they're potentially changing.

So we're really not able to say at this time whether or not the results of these tornadoes and flooding this year is directly related to climate change. It's certainly consistent with some of our projections going forward, but we're really not able to say with some degree of high certainty whether or not that's the case at this time.

William Brangham:

My understanding is, there's been some reporting that climate change perhaps might be shifting where certain tornadoes might be hitting, hitting places that are not used to them.

Is that right?

Victor Gensini:

That's exactly right.

We did a study last October that looked at, well, the number of tornadoes across the United States really hasn't changed very much over the last 40 years, but where they're happening has been changing.

So places like Tornado Alley, Texas, Oklahoma and Kansas, we have been seeing a downward trend in some of those locations, and an increasing trend further to east in places like Mississippi, Alabama, Tennessee, and portions of the Midwest, which is a big deal for population density and vulnerability, of course, the exposure, as we have many more people living east of the Mississippi River.

William Brangham:

I know you also do a lot of prediction work as well.

We're seeing this incredible heat wave. We know there's some rains projected. Over the next few days and weeks, what does it look like for these people who are suffering so much already?

Victor Gensini:

Thankfully, it looks like a reprieve is on the horizon.

Today, day 13, looks like it will be the last day of significant tornado activity, at least in this string, this rash, if you will, of severe weather. Things look like they will calm down, especially by the time we get into the weekend and early next week.

So, we have some reason to believe that things will calm down and be much quieter, which I'm sure the victims of these horrible tornadoes in the cities will be very thankful that they can really begin the cleanup efforts.

William Brangham:

That is good news.

And, of course, we will be watching out for all those people who might be suffering some flooding in the next couple of days.